Joints and Joining Methods for the Heat Transfer Fluid Circuit of Trough-Type Solar Collector Systems

ABSTRACT

Embodiments disclosed herein include flexible joints configured to be positioned between the movable and stationary elements of a CSP heat transfer fluid circuit. Other embodiments include parabolic trough solar reflector modules, solar collectors or solar collector loops having joints between the movable and stationary elements of the heat transfer fluid circuit including at least one or more flexible pipes comprising a loop segment defining at least a partial loop around the axis of rotation.

RELATED APPLICATIONS

This application claims the benefit under 35 USC section 119 of U.S. provisional application 61/983,815 filed on Apr. 24, 2014 and entitled “Joints and Joining Methods for the Heat Transfer Fluid Circuit of Trough-Type Solar Collector Systems,” the content of which is hereby incorporated by reference in its entirety and for all purposes.

TECHNICAL FIELD

The apparatus and methods disclosed herein relate to the joints used between the moving and stationary portions of a heat transfer fluid circuit associated with some types of concentrated solar power collectors. In particular, the disclosed apparatus and methods relate to flexible joints between the receivers of parabolic trough collectors and the stationary portions of the heat transfer fluid circuit.

BACKGROUND

Concentrated Solar Power (CSP) systems utilize solar energy to directly or indirectly heat a working fluid which drives a thermal power cycle for the generation of electricity. Considerable interest in CSP has been driven by renewable energy portfolio standards applicable to energy providers in the southwestern United States and renewable energy feed-in tariffs in Spain. CSP systems are typically deployed as large, centralized power plants to take advantage of economies of scale.

One type of CSP system uses multiple parabolic trough reflectors to concentrate sunlight on a receiver containing a heat transfer fluid. The thermal energy of the heat transfer fluid is then transferred to a working fluid such as steam and used to drive a power generation cycle. Conventional parabolic trough-based CSP systems typically include a several lengthwise arrays of individual trough reflector elements known as modules. The lengthwise array of multiple modules is commonly referred to as a collector. A representative collector 10 including twelve separate modules 12 is illustrated in FIG. 1. A more detailed view of a single module 12 is included in FIG. 2. Individual modules 12 and each collector 10 are usually aligned on a north-south axis and the reflective surfaces and supporting structures are rotated to track the sun as it moves across the sky each day.

Each module includes a supporting frame 14 and multiple reflecting elements 16. The supporting frames 14 are typically connected end-to-end and ultimately connected to a drive mechanism illustrated as a central drive 18 in FIG. 1. The drive 18 rotates the modules 12 around a lengthwise axis throughout the day to track the sun. The direction of the lengthwise axis is marked as “L” on FIGS. 1 and 2. The surfaces of the reflecting elements 16 are formed in a parabolic curve within a plane perpendicular to the lengthwise axis, illustrated as direction. “W” on FIGS. 1 and 2. The reflecting elements 16 are implemented with highly reflective metal or metallized glass facets. Therefore, the energy of incident sunlight (represented by a dashed arrow in FIG. 2) is focused along a lengthwise zone of concentrated solar flux which corresponds with the position of a solar receiver 20.

The solar receiver 20 can be implemented as a tube having surface characteristics making it suitable for absorbing solar energy. A heat transfer fluid, for example thermal oil or a molten salt is flowed within a series of receivers thereby causing the heat transfer fluid to be heated to an operational temperature. Thermal energy stored and transported in the heat transfer fluid is subsequently flowed through other portions of the heat transfer fluid circuit and utilized to generate electrical energy.

The length of each collector 10 is limited by the capacity of the drive 18 and the strength and torsional stiffness of each module 12. The heat transfer fluid will typically be required to pass through several collectors connected in series to reach a suitable hot temperature. This group of collectors connected in series between cold and hot heat transfer fluid supply piping is known as a “loop”.

As noted above, each module in a collector or loop must be rotated to track the sun. Therefore, each receiver 20 is moved along an arc as the associated module is rotated around the lengthwise axis of rotation. On the contrary, supply and return portions of the heat transfer fluid circuit are stationary. Typically, a ball joint or a combination of a flexible hose and a rotary joint is used to connect the first and last receiver tubes of a loop to adjacent stationary heat transfer fluid supply and return piping. Similar ball or rotary joint connections are often used between each collector in the loop to allow for thermal expansion of the receiver tubes and to enable each collector to move independently. For example, FIG. 3 shows a rotary joint 22 and flexible hose 24 between adjacent modules which allow these modules to be rotated independently around a lengthwise axis.

A ball and socket joint 26, as illustrated in FIG. 4, allows both rotation and limited angular movement. It may be noted from FIG. 4 that a typical ball and socket joint 26 includes a ball surface 28 and a socket surface 30 that are mated with a seal 32 such that the ball surface 28 and socket surface 30 may be rotated or moved with respect to each other while the seal 32 maintains fluid-tight engagement with each surface. Several such joints may be combined into a ball joint assembly 34 as shown in FIG. 5 to accommodate thermal expansion of the receiver and to provide for rotation as is required between the last receiver 20 of a loop and stationary heat transfer fluid supply piping 36.

An alternative joining method, illustrated in FIG. 6, includes the flexible hose 24 of FIG. 3 to provide for receiver expansion and a rotary joint 22 facilitating rotation. In the FIG. 6 configuration, the rotary joint 22 is placed in-line with the rotation axis of the collector and allows the receiver 20 and flexible hose 24 to move as the collector tracks the sun. The joint embodiments of FIG. 5 and FIG. 6 are suitable for use with lower temperature heat transfer materials such as conventional heat transfer oils.

However, the seal materials of ball joints and rotary joints are not compatible with certain heat transfer fluids of particular interest for use in CSP systems, high temperature salts for example. Commonly available seal materials for ball joints or rotary joints can fail or combust at the high temperature of a molten salt heat transfer fluid. In addition, the freezing point of a high-temperature heat transfer fluid will set a lower bound on the minimum operating temperature which must be maintained in a ball or rotary joint before the heat transfer fluid freezes causing leakage, blockage and another undesirable events. Because highly desirable heat transfer materials or fluids such as molten salts freeze at temperatures above normal ambient temperatures, the initial filling of a heat transfer fluid circuit and the night-time maintenance of the system require the heat transfer fluid circuit to be heated with a supplemental means at times when solar heating is unavailable. The heating of typical ball joints or rotary joints present difficult and costly challenges.

The embodiments disclosed herein are directed toward overcoming one or more of the problems discussed above.

SUMMARY OF THE EMBODIMENTS

Certain embodiments disclosed herein are flexible joints configured to be positioned between the movable and stationary elements of a CSP heat transfer fluid circuit. Other embodiments include parabolic trough solar reflector modules, solar collectors or solar collector loops having joints between the movable and stationary elements of the heat transfer fluid circuit which do not require ball joints, rotary joints or other joints having mating seal surfaces which slide, rotate or move with respect to each other.

One particular embodiment is a solar collector comprising a linear array of one or more parabolic trough reflectors. Each parabolic trough reflector in the collector array is configured to reflect sunlight to a receiver positioned in an elongated zone of concentrated solar flux. Each reflector includes one or more support frames which provide for the reflector and receiver elements to be rotated around an axis of rotation substantially parallel to, but typically displaced from, the elongated zone of concentrated solar flux. The rotational motion allows the reflector to track the motion of the sun and thereby maintain focus of concentrated solar flux on the receiver.

The solar collector also includes or is associated with a heat transfer fluid circuit. The heat transfer fluid circuit comprises at least one stationary cold-side supply pipe which transports relatively cool heat transfer fluid to the collector. The heat transfer fluid circuit also includes at least one stationary hot-side return pipe or other return pipe which transports heated heat transfer fluid away from the collector. Heat transfer fluid flows from the supply pipe to the return pipe through the various joints and solar receivers described herein. As the heat transfer fluid is flows through the receiver elements, it is heated to an operational temperature by concentrated solar flux. The connection between the stationary supply or return pipes and the input to or output from the first and last receivers in the collector must accommodate the motion of the receivers as the parabolic trough reflectors are rotated around the axis of rotation to track the sun.

Accordingly, a joint is provided between the stationary supply and/or return pipes and the input to or the output from the adjacent receivers. The joint comprises a flexible pipe connecting the stationary heat transfer fluid circuit pipe with the inlet or outlet of a receiver. The flexible pipe may comprise a loop segment defining at least a partial loop around the axis of rotation. In certain embodiments the loop segment is supported at least in part by a drum approximately centered upon the axis of rotation. In other embodiments, the loop segment is self-supporting and may be configured as a freestanding coil.

In certain embodiments, the joint and in particular the flexible pipe portion of the joint is fabricated at least in part from an electrically conductive material. The electrically conducting material may be, but is not limited to, corrugated pipe with a stainless steel overbraid or a stainless steel pipe configured in a freestanding coil. Providing an electrically conductive joint between a stationary heat transfer fluid circuit pipe and a receiver of the parabolic trough reflectors facilitates the direct impedance heating of each of the pipes, joints, receivers or other elements of the heat transfer fluid circuit with a reduced number of transformers.

Therefore, the disclosed embodiments are particularly well-suited for implementation with heat transfer fluids which have a freezing temperature which is above the ambient temperature experienced by the system at night, or during extended cloudy periods. The disclosed joints are particularly well-suited for implementation with systems utilizing a molten salt heat transfer fluid which might be unsuitable for use with conventional ball or rotary joint assemblies positioned between a solar receiver and stationary heat transfer fluid circuit piping because of the potential for molten salt to burn or destroy conventional joint packing materials and further in view of the difficulty conducting electricity across a ball or rotary joint having conventional seal materials.

Accordingly, in certain embodiments the disclosed solar collector or loop will include one or more transformers in electrical communication with stationary heat transfer fluid circuit piping, joints and receivers to provide for the direct impedance heating of these elements.

Certain alternative embodiments include fluid tight joints providing for the connection of a stationary pipe to the receiver of a parabolic trough solar reflector. The disclosed joints include a flexible pipe connecting the stationary pipe to a receiver inlet or outlet and no ball, rotary or other joints having mating sealing surfaces which rotate, slide or move with respect to each other between the stationary piping and receiver. The flexible pipe of the joint may include a loop segment which extends through any degree of loop rotation, for example, a loop rotation of at least 360°. In some embodiments the joint includes a drum supporting at least a portion of the loop segment. In other embodiments, the loop segment is implemented with a freestanding coil.

Some portion or the entire flexible pipe element may be fabricated from an electrically conductive material such that electrical connectivity may be maintained between the stationary pipe connection and the receiver connection. For example, the flexible pipe may include a corrugated inner hose and a stainless steel overbraid or the flexible pipe may be implemented with a coiled stainless steel pipe segment.

Alternative embodiments include methods of joining the receiver of a solar collector to stationary heat transfer fluid circuit piping using the joints described herein.

Alternative methods include providing direct impedance heating to a solar collector or loop by flowing current through conductive pipes, joints, receivers or other elements in a heating circuit which extends across one or more of the described joints.

Alternative embodiments include systems and methods for generating electricity utilizing a thermal power cycle. In system embodiments, thermal energy is provided directly or indirectly to a working fluid from solar flux concentrated using parabolic trough collectors having joints as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan and front elevation view of a prior art collector comprised of multiple parabolic reflector modules having a central drive system.

FIG. 2 is a isometric in view of one parabolic trough reflector module.

FIG. 3 is a front isometric view of portions of two adjacent parabolic trough reflectors in a prior art collector featuring flexible tubing and a rotary joint between adjacent receiver elements.

FIG. 4 is a cross-sectional view of a prior art ball joint.

FIG. 5 is a front elevation view of a prior art joint between a receiver and stationary heat transfer fluid circuit piping using three ball joints to accommodate receiver expansion and motion.

FIG. 6 is a front elevation view of a prior art joint between a receiver and stationary heat transfer fluid circuit piping using flexible tubing and a rotary joint to accommodate receiver expansion and motion.

FIG. 7 is a simplified schematic diagram illustration of a concentrated solar power system featuring a parabolic trough collector.

FIG. 8 is an isometric view of a joint between a receiver and stationary heat transfer fluid circuit piping as disclosed herein.

FIG. 9 is a front elevation view of the joint of FIG. 8.

FIG. 10 is a side elevation view of the joint of FIG. 8.

FIG. 11 is an isometric view of another joint between a receiver and stationary heat transfer fluid circuit piping as disclosed herein.

FIG. 12 is a front elevation view of the joint of FIG. 10.

FIG. 13 is a side elevation view of the joint of FIG. 10.

FIG. 14 is a simplified schematic diagram of a collector loop featuring joints and impedance heating across multiple loop elements as described herein.

DETAILED DESCRIPTION

Unless otherwise indicated, all numbers expressing quantities of ingredients, dimensions, reaction conditions and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about”.

In this application and the claims, the use of the singular includes the plural unless specifically stated otherwise. In addition, use of “or” means “and/or” unless stated otherwise. Moreover, the use of the term “including”, as well as other forms, such as “includes” and “included”, is not limiting. Also, terms such as “element” or “component” encompass both elements and components comprising one unit and elements and components that comprise more than one unit unless specifically stated otherwise.

All the features described in this specification (including the claims, description and drawings) and/or all the steps of the described method can be combined in any combination, with the exception of combinations of mutually exclusive features and/or steps.

As noted above, concentrated solar power (CSP) plants utilizing parabolic trough reflectors as the primary solar energy concentrating element typically include a large number of individual parabolic trough reflector modules arranged in groups or arrays. Each reflector module usually consists of several reflective facets supported by framework. As shown in FIG. 1, several modules are typically arranged in a linear group known as a collector. Multiple collectors may be connected in series to define a heat transfer fluid loop. As used herein, the terms, “a linear array of parabolic trough reflectors” includes any linear array of at least two reflector modules including but not limited to a collector, a loop or other array of individual reflectors or modules.

CSP systems such as described herein utilize concentrated sunlight to directly or indirectly heat a working fluid which is used to drive one or more power generation cycles. Many CSP systems include an initial heat transfer fluid circuit where heat transfer fluid is directly heated to high operational temperatures by concentrated solar energy. The heat transfer fluid circuit exchanges heat with a separate power cycle working fluid circuit. For example, FIG. 7 illustrates a highly simplified CSP system which for ease of illustration includes only a single parabolic trough reflector. The CSP system 42 features a receiver 20 situated at or within the longitudinal zone of concentrated solar flux of a parabolic trough reflector 46. As noted above, any commercial implementation would include a large number of individual parabolic trough elements arranged in linear arrays of collectors and loops.

A primary heat transfer fluid circuit 48 carries heat transfer fluid through the receivers 20 where the heat transfer fluid is heated to an operational temperature. Thermal energy from the heat transfer fluid may be stored at any point in the heat transfer fluid circuit, for example in thermal energy storage devices 50 or 52 to extend the operational timeframe of the system.

In the simplified diagram of FIG. 7, the heated heat transfer fluid is conveyed to a heat exchanger 54 from the receiver 20 or from TES 50 where thermal exchange causes the heating of a working fluid such as steam flowing in a working fluid circuit 55.

The thermal energy of the working fluid in the working fluid circuit 55 is utilized to drive a thermal power cycle 56. In the particular embodiment of FIG. 7, the thermal power cycle 56 is schematically represented as a highly simplified Brayton cycle featuring a turbine 58 and compressor 60 connected by an axle 62. Expansion of the heated and pressurized working fluid within the turbine 58 converts thermal energy to mechanical energy, thereby outputting work (represented by rotational arrow 64) which can be utilized to drive an electric generator. Downstream from the turbine 58, surplus heat is used in a recuperator 65 to preheat the working fluid prior to the heat exchanger 54. Additional heat (represented by arrow 66) is rejected from the system through an air-cooled condenser 68. Other types of thermal power cycle or heat engines of any level of complexity may also be driven with thermal energy obtained initially from solar flux concentrated with a linear array of parabolic trough reflectors 46.

As noted above, to best track the sun and most effectively concentrate solar flux, each parabolic reflector element must pivot or rotate around an axis which is substantially parallel to, but typically displaced from, the axis defined by the elongated receiver 20. On the contrary, the hot and cold heat transfer fluid pipework (schematically illustrated as heat transfer fluid circuit 48 on FIG. 7) is typically stationary pipe and ductwork. Therefore, a joining method is required which accommodates both thermal expansion and the motion between the receiver inlet and outlet while connected to the stationary portions of the heat transfer fluid circuit.

Various embodiments disclosed herein include one or more parabolic solar collectors, having joints which provide for motion between a receiver input and receiver output and the stationary portions of a heat transfer fluid circuit. In each embodiment, no ball joints, rotary joints or other joints having a fluid-tight sealing surface that rotates or slides with respect to a mating fluid tight sealing surface is used between the stationary heat transfer fluid circuit pipes and the receiver.

The scope of the present disclosure includes large or small and simple or complex systems. For example, the disclosed joints could be implemented between a single parabolic trough reflector having a single receiver and adjacent stationary heat transfer fluid circuit supply or return pipes. In any commercial embodiment however, it is much more likely that a described joint will be implemented between a stationary heat transfer fluid circuit pipe and the inlet to a first receiver or the outlet to a final receiver in a linear collector or loop array of multiple parabolic trough reflectors.

The embodiments disclosed herein are particularly well-suited for use with certain high temperature heat transfer fluids. Heat transfer fluids that are stable at very high temperatures (for example, above 400° C.) can improve the efficiency of CSP systems by increasing the steam temperature generated in the working fluid circuit 55. One problem with many high temperature heat transfer fluids is that certain materials, such as molten salts, freeze at relatively high temperatures, which are well above the ambient temperature of the CSP system at night or when the system is initially filled. For example, nitride salts which are highly desirable heat transfer fluids freeze at 150° C.-260° C. Therefore, when implementing a CSP system to utilize a molten salt heat transfer fluid, a freeze protection system is required to keep the heat transfer fluid from solidifying in the receiver tubes, joints, circuit piping or other elements at ambient temperatures.

As described in detail below, one method of providing freeze protection (or to allow the initial filling of a system) is to electrically heat the receiver elements, supply and return piping and joints. For example, joints and stationary piping could be wrapped or otherwise associated with electrical heating elements. A typical receiver cannot be implemented with an external impedance heater however, because the heater elements and associated insulation would block sunlight during use and heater materials suitable for external impedance heating would be very expensive when engineered to withstand the extreme temperatures of the receiver during use. Accordingly, it can be useful to construct the receiver and heat transfer fluid piping from materials, such as high temperature stainless steel, which have sufficient conductivity to operate directly as impedance heating elements when electrical current is applied to some portion or all of a heat transfer fluid loop.

Conventional ball and rotary joints such as illustrated in FIGS. 3-6 have distinct shortcomings when implemented in a CSP system utilizing a molten salt or other heat transfer fluid materials having a relatively high freeze point. For example, the conventional packing material seals utilized with rotary or ball joints will typically not withstand the high operational temperatures of a molten salt heat transfer fluid without combusting or leaking. In addition, it is difficult or impossible to flow electrical current through the elements of a rotary or ball joint to provide direct impedance heating because the seal acts as a nonconductive break in any impedance heating circuit, requiring the use of additional transformers at additional cost.

The problems noted above may be addressed by implementing CSP systems having joints and joining methods as illustrated in FIGS. 8-13. Each of FIGS. 8-13 shows a joint between a stationary heat transfer fluid supply or return pipe and the inlet or outlet to or from a receiver.

One particular embodiment of joint 70 is shown in FIGS. 8-10. The joint 70 includes a flexible pipe 72 connecting a stationary heat transfer fluid supply or return pipe 74 to the inlet or outlet of a receiver 20 associated with a parabolic trough reflector 78. The flexible pipe 72 may be implemented with any suitable high temperature pipe material which exhibits the necessary flexibility to operate as described herein. For example, in certain embodiments, the flexible pipe may be implemented with a corrugated high-temperature hose having a stainless steel overbraid. In embodiments where the flexible pipe element 72 is fabricated at least in part with conductive materials, for example a stainless steel overbraid, conductivity may be maintained between the receiver 20 and stationary heat transfer fluid circuit supply or return piping 74 providing for direct impedance heating as described in detail below.

In use, as the system tracks the sun, the receiver 20 will be moved through a substantial arc around an axis of rotation defined by the supporting frame 14 supporting the reflective facets of the trough reflector 78. Therefore, the flexible pipe 72 must accommodate substantial motion of the receiver 20 with respect to the stationary heat transfer fluid piping 74 on a daily basis without undue failure. This requirement may, in certain embodiments, be facilitated by providing the flexible pipe 72 with a whole or partial loop 82 substantially positioned around the axis of rotation. In the embodiment illustrated in FIGS. 8-10, the axis of rotation is concentric with a drum 84 which supports at least a portion of the loop 82 formed in the flexible pipe 72. Thus, when the parabolic reflector pivots, the flexible pipe 72 may coil or uncoil from the drum 84 which is providing support for a portion of the flexible pipe 72.

The particular configuration illustrated in FIGS. 8-10 includes a flexible pipe 72 having two distinct segments. The first segment 86 extends from the inlet or outlet of the receiver 20 to a coupling 88 which in use is positioned near the perimeter of the drum 84. The second flexible pipe segment 90 extends from the coupling 88 around the drum 84 and to the stationary supply or return side heat transfer fluid circuit pipe 74. As is best seen in FIG. 10, this configuration facilitates the coiling or uncoiling of the flexible pipe 72 around the drum 84 while minimizing undue flexure of the first flexible pipe segment 86 as the receiver 20 is moved through the daily arc. The first flexible pipe segment 86 accommodates thermal expansion of the receiver 20. The second flexible pipe segment 90 includes a lower loop 92 which eases the transition to the stationary pipe 74 and prevents over flexing of the second segment 90 as the flexible pipe 72 coils or uncoils around the drum 84.

The joint 70 therefore includes a flexible pipe 72 between the inlet or outlet of a receiver 20 and stationary heat transfer fluid circuit piping 74 which accommodates the necessary motion between the receiver and the stationary piping without requiring any conventional joint such as a ball joint, rotary joint or other joint having a fluid tight sealing surface that rotates, slides or moves across a mating fluid tight sealing surface. Therefore, leakage is prevented, seal packing material degradation or destruction is prevented and, as is described in detail below, an electrical current can flow from the receiver to the heat transfer fluid piping facilitating the impedance heating of the joint 70 and adjacent elements.

An alternative joint 94 configuration is illustrated in FIGS. 11-13. In this alternative embodiment, the joint 94 also includes a flexible pipe 96 connecting the inlet or outlet of a receiver 20 with stationary heat transfer fluid circuit piping 100. Joint 94 however includes a self-supporting coil 102 as a portion of the flexible pipe 96. In particular, the flexible pipe 96 comprises a first segment 104 extending from the receiver 20 to a coupling 106. The self-supporting coil portion 102 extends from the coupling 106 to the stationary supply or return pipe 100 and, as shown in FIG. 13, the coiled portion 102 may be approximately centered on the axis of rotation. Thus, when the receiver 20 is moved through an arc, the coil 102 uncoils or coils more tightly to accommodate relative motion between the receiver 20 and stationary pipe 100. The self-supporting coil 102 may define a loop having any dimension and extending through any number of rotations. For example, the coil may define a loop of at least 360° or more. The coil may be implemented with any suitable material, including but not limited to stainless steel pipe.

As noted above, joints 70 or 96 are typically implemented at the inlet to a first receiver or the outlet from the last receiver in a collector, loop or linear array. In this configuration, crossover pipes may be used to connect the outlets and inlets of adjacent interior receivers. The crossover pipes may be implemented with flexible tubing or other means to accommodate thermal expansion between the receivers. However, substantially rigid crossover pipes do require that each parabolic reflector in an array be rotated in unison to track the sun. Unified rotation may be accomplished by mechanically coupling the modules of adjacent reflectors and providing centralized or distributed drive mechanisms to rotate the entire array together. Alternatively, optical, mechanical or other sensors may be implemented between adjacent modules to coordinate and control unified rotation. Unified rotation between adjacent modules eliminates the requirement for joints such as the prior art rotary joint 22 (FIG. 3) or joints 70 or 94 as disclosed herein between the interior receivers of an array. However, if independent rotation between the receivers of interior parabolic reflectors in an array is desired, joints 70 or 94 may be implemented to connect any two adjacent receivers.

The joints 70 or 94 described herein also facilitate impedance heating across multiple parabolic trough reflectors and therefore across some or all of a collector or loop. For example, FIG. 14 is a schematic illustration of a highly simplified loop 108 featuring impedance heating across the entire loop. The loop 108 includes two linear collectors 110 and 112 comprised of multiple parabolic trough reflectors 114. The inlet to the receiver of the first parabolic trough reflector 114A in the loop is connected to stationary cold-side heat transfer fluid circuit supply piping 116 with a joint 118 such as joint 70 or 94 described above. The receivers of adjacent inside reflectors are connected with crossover pipes 120 as described above. Therefore, each of the reflectors in a collector (for example reflectors 114A, 114 B and 114C in collector 110) are caused to rotate together when tracking the sun. The outlet of reflector 114C is connected to the inlet of reflector 114D of collector 112 using joints 122 and 124 as described herein connected to an extended crossover pipe 126. Similarly, the outlet of the receiver associated with reflector 114F is connected through joint 128 to the hot-side return pipe 130 of the heat transfer circuit.

Impedance heating is provided across all elements of the loop 108 by a single transformer 132 in electrical communication with the supply and return pipes 116 and 130. Thus, electrical current may be caused to flow from the transformer 132 through the various joints, receivers and crossover pipes without interruption by insulating materials such as the seals or bearing packing materials of conventional rotary or ball joints. In alternative embodiments, more than one transformer may be provided if required to provide suitable current for impedance heating.

Various embodiments of the disclosure could also include permutations of the various elements recited in the claims as if each dependent claim was a multiple dependent claim incorporating the limitations of each of the preceding dependent claims as well as the independent claims. Such permutations are expressly within the scope of this disclosure.

While the embodiments disclosed herein have been particularly shown and described with reference to a number of alternatives, it would be understood by those skilled in the art that changes in the form and details may be made to the various configurations disclosed herein without departing from the spirit and scope of the disclosure. The various embodiments disclosed herein are not intended to act as limitations on the scope of the claims. All references cited herein are incorporated in their entirety by reference. 

What is claimed is:
 1. A solar collector comprising: a linear array of one or more parabolic trough reflectors wherein each parabolic trough reflector in the linear array is configured to reflect sunlight to a receiver positioned in an elongated zone of concentrated solar flux defined by the parabolic trough reflector; one or more support frames with at least one support frame supporting each parabolic trough reflector in the linear array and providing for the supported parabolic trough reflector to be rotated around an axis of rotation parallel to the elongated zone of concentrated solar flux; and a heat transfer fluid circuit comprising; a stationary supply pipe; a first flexible pipe connecting the stationary supply pipe to the inlet of a first receiver associated with the first parabolic trough reflector in the linear array; one or more crossover pipes connecting the outlets of the receivers of the parabolic trough reflectors in the linear array to the inlets of the receivers of adjacent parabolic trough reflectors; and a second flexible pipe connecting the outlet of a last receiver associated with the last parabolic trough reflector in the linear array to a stationary return side pipe, wherein the first flexible pipe provides for heat transfer fluid to flow from the stationary supply pipe to the first receiver and the second flexible pipe provides for heat transfer fluid to flow from the last receiver to the stationary return pipe, and wherein the first and second flexible pipes comprise a loop segment defining at least a partial loop around the axis of rotation.
 2. The solar collector of claim 1 wherein the loop segment defines a loop of at least 360°.
 3. The solar collector of claim 1 wherein the loop segment of the first and second flexible pipes is supported at least in part by a drum.
 4. The solar collector of claim 3 wherein the drum is centered upon the axis of rotation.
 5. The solar collector of claim 1 wherein the stationary supply pipe, first flexible pipe, each receiver, each crossover pipe, the second flexible pipe and the stationary return pipe are electrically conductive and electrical conductivity is maintained throughout the heat transfer circuit.
 6. The solar collector of claim 5 further comprising at least one transformer in electrical communication with the heat transfer fluid circuit and providing for electrical current flow within the heat transfer fluid circuit sufficient to cause impedance heating of the stationary supply pipe, first flexible pipe, each receiver, each crossover pipe, the second flexible pipe and the stationary return pipe.
 7. The solar collector of claim 6 comprising two or more parabolic trough reflectors in the linear array and wherein no more than one transformer is associated with the heat transfer fluid circuit.
 8. The solar collector of claim 1 wherein the first and second flexible pipes comprise a corrugated hose with a stainless steel overbraid.
 9. The solar collector of claim 1 wherein the first and second flexible pipes comprise a coiled stainless steel pipe segment.
 10. The solar collector of claim 1 further comprising a molten salt heat transfer fluid having a freezing temperature greater than 0° C.
 11. A fluid tight joint providing for the connection of a stationary pipe to the receiver of a parabolic trough solar reflector comprising: a flexible pipe; a stationary pipe connection at a first end of the flexible pipe; and a receiver connection at a second end of the flexible pipe wherein the flexible pipe comprises a loop segment defining at least a partial loop between the stationary pipe connection and the receiver connection.
 12. The fluid tight joint of claim 11 wherein the loop segment defines a loop of at least 360°.
 13. The fluid tight joint of claim 11 further comprising a drum supporting at least a portion of the loop segment.
 14. The fluid tight joint of claim 11 wherein the flexible pipe is electrically conductive and electrical conductivity is maintained from the stationary pipe connection and the receiver connection.
 15. The fluid tight joint of claim 11 wherein the flexible pipe comprises a corrugated inner hose and a stainless steel overbraid.
 16. The fluid tight joint of claim 11 wherein the flexible pipe comprises a coiled stainless steel pipe segment.
 17. A method of joining a solar collector to stationary piping comprising: providing a linear array of one or more parabolic trough reflectors wherein each parabolic trough reflector in the linear array is configured to reflect sunlight to a receiver positioned in an elongated zone of concentrated solar flux defined by the parabolic trough reflector; providing one or more support frames with at least one support frame supporting each parabolic trough reflector in the linear array and providing for the supported parabolic trough reflector to be rotated around an axis of rotation parallel to the elongated zone of concentrated solar flux; providing a heat transfer fluid circuit comprising; a stationary supply pipe; a first flexible pipe connecting the stationary supply pipe to the inlet of a first receiver associated with the first parabolic trough reflector in the linear array; one or more crossover pipes connecting the outlets of the receivers of the parabolic trough reflectors in the linear array to the inlets of the receivers of adjacent parabolic trough reflectors; and a second flexible pipe connecting the outlet of a last receiver associated with the last parabolic trough reflector in the linear array to a stationary return side pipe, wherein the first and second flexible pipes comprise a loop segment defining at least a partial loop around the axis of rotation; and flowing heat transfer fluid from the stationary supply pipe to the first receiver, and flowing heat transfer fluid from the last receiver to the stationary return pipe, while the receivers are rotated in an arc around the axis of rotation.
 18. The method of claim 17 wherein the loop segment defines a loop of at least 360°.
 19. The method of claim 17 further comprising supporting the loop segment of the first and second flexible pipes at least in part with a drum.
 20. The method of claim 19 wherein the drum is centered upon the axis of rotation.
 21. The method of claim 17 wherein the stationary supply pipe, first flexible pipe, each receiver, each crossover pipe, the second flexible pipe and the stationary return pipe are electrically conductive and the method further comprises maintaining electrical conductivity throughout the heat transfer circuit.
 22. The method of claim 21 further comprising providing at least one transformer in electrical communication with the heat transfer fluid circuit and providing for electrical current flow within the heat transfer fluid circuit sufficient to cause impedance heating of the stationary supply pipe, first flexible pipe, each receiver, each crossover pipe, the second flexible pipe and the stationary return pipe.
 23. The method of claim 22 further comprising providing two or more parabolic trough reflectors in the linear array and providing no more than one transformer in electrical communication with the heat transfer fluid circuit
 24. The method of claim 17 further comprising providing first and second flexible pipes comprising a corrugated hose with a stainless steel overbraid.
 25. The method of claim 17 further comprising providing first and second flexible pipes comprising a coiled stainless steel pipe segment.
 26. The method of claim 17 further comprising providing a molten salt heat transfer fluid having a freezing temperature greater than 0° C. 